Method for cooling a hydrocarbon-rich fraction

ABSTRACT

A method is described for cooling a hydrocarbon-rich fraction, in particular of natural gas, wherein
         the hydrocarbon-rich fraction ( 1 ) is cooled against at least one coolant circuit ( 10 - 15 ),   the coolant has at least nitrogen, carbon dioxide, methane and/or C 2+  hydrocarbons,   the coolant is compressed by means of at least one turbocompressor (C 1 ) having one or more gas-lubricated sliding-ring seals, and   as primary seal gas, a substream of the coolant and/or an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor (C 1 ), and as secondary seal gas, nitrogen is fed.       

     According to the invention, at least one nitrogen-rich stream ( 21 ) is withdrawn at least at times from the coolant circuit ( 10 - 15 ).

The invention relates to a method for cooling a hydrocarbon-rich fraction, in particular of natural gas, wherein

-   -   the hydrocarbon-rich fraction is cooled against at least one         coolant circuit, the coolant has at least nitrogen and/or carbon         dioxide and/or methane and/or C₂₊ hydrocarbons,     -   the coolant is compressed by means of at least one         turbocompressor having one or more gas-lubricated sliding-ring         seals, and     -   as primary seal gas, a substream of the coolant and/or an         external gas or gas mixture having substantially nitrogen and/or         methane is fed to the turbocompressor, and as secondary seal         gas, nitrogen is fed.

Methods of the type in question for cooling a hydrocarbon-rich fraction are used, in particular in the liquefaction of natural gas. Cold is required for the liquefaction process, which cold is usually provided by one or more coolant circuits. Here, closed coolant circuits which are driven by turbocompressors are of particular importance. As coolants, in closed circuits, sometimes pure substances are used, but mostly material mixtures using a selection of material components nitrogen, methane and C₂H₄, C₂H₆, C₃H₆, C₃H₈, i/n-C₄H₁₀ and i/n-C₅H₁₂, etc. in variable fractions. The expression “C₂₊ hydrocarbons”, in the present case, may be taken to mean the abovementioned components C₂H₄, C₂H₆, C₃H₆, C₃H₈, i/n-C₄H₁₀ and i/n-C₅H₁₂, etc.

For the steady-state plant operation, the inventory of the cold circuit with respect to amount and molar composition is kept constant. However, inevitably leaks of the cold circuit occur, or the entry of additional substance streams or component streams into the cold circuit occurs. Components that are substantially responsible therefor are the shaft seals of the turbocompresor provided for compressing the coolant, and also the seal gas supply thereof.

This contamination and/or leakage of the coolant must be compensated for. Whereas, usually, the availability of nitrogen within a liquefaction plant and also the provision of methane from the natural gas that is to be liquefied is ensured, the permanent compensation of the C₂₊ hydrocarbon losses is associated with a high installation expenditure, high operating costs and possibly logistical problems.

It is therefore necessary to ensure that, in the selection of the shaft seal of the turbocompressor, and also in the design of the associated peripherals for sealing, the coolant inventory of the cold circuit is protected as well as possible. For this purpose, currently, predominantly gas-lubricated sliding-ring seals of various types are used. As primary seal gas, these are charged with the coolant circulating in the associated cold circuit, in order to avoid process-side contamination. The secondary sealing, for reasons of operational safety, is always performed with nitrogen, and so in the primary outlet line of the gas seal, a mixture of coolant and nitrogen is present. This mixture is generally removed to the plant flare, from which a loss of coolant results.

In principle, for the primary seal gas charging, an external seal gas can also be used, but in this case, even with suitable selection of the type of seal, a foreign component entry into the cold circuit and therefore contamination of the coolant is always present. Since contamination, in some circumstances, leads to a loss of relatively large amounts of the inventory, usually the sealing by means of coolant is preferred and the sealing losses reduced by suitable controlling, in particular of the comparatively valuable C₂₊ hydrocarbons and C₂₊ components are accepted.

it is the object of the present invention to specify a method of the type in question for cooling a hydrocarbon-rich fraction, which avoids the abovementioned disadvantages.

To achieve this object, a method for cooling a hydrocarbon-rich fraction is proposed which is characterized in that at least one nitrogen-rich stream is withdrawn at least at times from the coolant circuit.

In this case, the nitrogen-rich stream is preferably withdrawn at the cold end of the coolant circuit. The nitrogen-rich stream is withdrawn in particular when the nitrogen and/or methane content of the coolant circulating in the coolant circuit exceeds a preset threshold value.

With the procedure according to the invention, an enrichment of the cold circuit with nitrogen and/or methane can be avoided. The desired ratio between these components and the C₂₊ hydrocarbons remains substantially unchanged thereby. For controlling the composition of the coolant, it is therefore sufficient if merely these two components are replenished in the amount or amounts desired.

Advantageously, the nitrogen-rich stream that is withdrawn has a fraction of C₂₊ hydrocarbons of less than 2 mol %, preferably less than 0.5 mol %.

In comparison with the methods included in the prior art, the method according to the invention for cooling a hydrocarbon-rich fraction permits a reduction in the losses of C₂₊ hydrocarbons or C₂₊ coolant components by approximately 99%. This results not only in a substantial reduction in the operating costs owing to minimized expenditure with respect to procurement and transport of C₂₊ hydrocarbons and also a reduction in the installation costs for providing these coolant components. In addition, unwanted emissions can be decreased, since no hydrocarbons are passed to the flare.

A further advantageous embodiment of the method according to the invention for cooling a hydrocarbon-rich fraction is characterized in that, provided that the hydrocarbon-rich fraction is liquefied, fed to at least one storage container and from this a boil-off gas fraction is taken off, the nitrogen-rich stream that is withdrawn is at least partially added to the boil-off gas fraction and/or is flared off. The expression “boil-off gas fraction” is taken to mean in this case the liquid natural gas (LNG) evaporating owing to the introduction of heat into a storage vessel, and the gas displaced during the introduction of LNG into the storage vessel.

In as much as the coolant circulating in the coolant circuit is compressed in a single or multistage manner, it is particularly advantageous if the seal gas mixture that is taken off from the turbocompressor via the primary outlet line of the gas seal is fed to the compressor or the first compressor stage by an elevation of the seal gas pressure on the suction side.

In as much as, as primary seal gas, an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor, advantageously this gas (mixture) is produced from a fraction existing and/or occurring within the cooling process. For this purpose, for example, a substream of the compressed boil-off gas fraction and/or a substream of the process nitrogen can be used.

The method according to the invention for cooling a hydrocarbon-rich fraction will be described in more detail hereinafter with reference to the exemplary embodiment shown in FIG. 1, which shows a natural gas liquefaction process.

Via line 1, the natural gas that is to be cooled and liquefied is fed to a heat exchanger E in which it is cooled against the coolant of a cold circuit, which will he considered in more detail hereinafter, and liquefied. After liquefaction has been performed and optionally subcooling, the liquefied natural gas (LNG) is fed via line 2 to a storage container S. Liquefied natural gas is withdrawn from the storage container S via the line 3. A boil-off gas fraction produced within the storage container S is taken off via line 4. preferably warmed in the heat exchanger E against the natural gas 1 that is to be cooled and subsequently delivered via line 5—optionally after previous compression by a boil-off gas compressor C2—as what is termed a fuel gas fraction via line 22, A substream of this fraction can be fed as primary seal gas via the line sections 23 and 18 to the turbocompressor C1 which is still to be described.

The coolant circulating within the cold circuit has, for example, the components nitrogen, methane and C₂₊ hydrocarbons. This coolant is compressed in a turbocompressor C1 designed as a single-stage or multistage compressor having one or more gas-lubricated sliding-ring seals. The coolant that is compressed to the desired circuit pressure is fed via line 10 to the heat exchanger E and therein cooled against itself. Via Sine 11, the cooled coolant is taken off from the heat exchanger E and cold-producingly expanded in the expansion valve a.

The expanded coolant is then fed via line 12 to a separator D1 and therein separated into a liquid C₂₊ hydrocarbon-rich fraction 13 and a gaseous fraction 14 substantially containing solely nitrogen and methane. The two abovementioned fractions are recombined immediately upstream of the heat exchanger E and conducted through the heat exchanger E in counterflow to the natural gas stream 1 that is to be cooled and also to the coolant stream 10 that is to be cooled. The coolant that is warmed in this case is subsequently fed via line 15 to a container D2 that is connected upstream of the turbocompressor C1. This container serves for separating any liquid components present in the warmed coolant stream 15; the gas fraction produced in the container D2 is fed to the turbocompressor C1 via line 16.

As an alternative to the abovedescribed procedure, the coolant is expanded at the cold end of the coolant circuit in two stages. The coolant that is expanded in the first expansion stage in the valve a is separated as described in the separator D1 into a liquid C₂₊ hydrocarbon-rich fraction 13 and a gaseous fraction 14 substantially containing solely nitrogen and methane, wherein the liquid C₂₊ hydrocarbon-rich fraction 13 is then expanded to the evaporation pressure of the coolant in the valve a which is shown dashed. This procedure has the advantage compared with the abovedescribed procedure in that the low-boiling components that are to be ejected are enriched in the gas phase in the first expansion. The process can become more selective thereby.

A substream of the coolant circulating in the cold circuit and/or an external gas or gas mixture which substantially comprises nitrogen and/or methane, is fed as primary seal gas to the turbocompressor C1. This feed of a substream of the coolant proceeds via line 17 in which a control valve c is arranged. External gas or gas mixture can be fed as primary seal gas to the turbocompressor C1 via line 18, in which a control valve d is likewise arranged. In addition, nitrogen or a nitrogen-rich fraction is fed to the turbocompressor C as secondary seal gas. For the sake of clarity, this is not shown in FIG. 1.

If then, enrichment of the components nitrogen and/or methane occurs within the cold circuit, these components can be withdrawn from the cold circuit via line 21 in which a control valve b is arranged. Providing the abovedescribed separator D1 ensures that the nitrogen-rich stream 21 that is withdrawn via line 21 from the cold circuit contains virtually no C₂₊ hydrocarbons. Losses thereof within the cold circuit are therefore negligible.

As shown in FIG. 1, it may be expedient to add the nitrogen-rich stream 21 to the boil-off gas fraction 4 that is withdrawn from the storage container S and to heat it together with this. Alternatively, or supplementarily thereto, the nitrogen-rich stream 21 can also be flared off.

Preferably, the nitrogen-rich stream 21 is withdrawn at the cold end of the cold circuit. However, as an alternative thereto, other withdrawal sites are also conceivable. In addition, the nitrogen-rich stream 21 can be withdrawn continuously or discontinuous. The nitrogen-rich stream 21 is withdrawn, in particular, as soon as the nitrogen and/or methane content of the coolant circulating in the coolant circuit exceeds a preset threshold value. For this purpose it is necessary to monitor the composition of the coolant.

In addition, the seal gas mixture that is taken off from the turbocompressor C1 via the primary outlet line of the gas seal thereof can be fed back to the compressor, or in the case of a multistage compression, to the first compressor stage on the suction side, preferably via the abovedescribed container D2; this is indicated in FIG. 1 by the line 20. The abovedescribed recycling of the seal gas mixture that is taken off via the primary outlet line upstream of the turbocompressor C1 or the first compressor stage thereof can proceed directly by elevating the primary seal gas pressure without additional expenditure in terms of apparatus. The abovedescribed recycling upstream of the turbocompressor or the first compressor stage thereof of the seal gas mixture that is taken off via the primary outlet line can also be achieved using an ejector or further compressor. As motive stream for the ejector, for this purpose, coolant from the pressure side of the turbocompressor C1 can be used.

It is emphasized explicitly that the procedure according to the invention can be implemented, or is logical, not only in combination with the cold circuit shown in FIG. 1, but rather the concept according to the invention can be implemented in combination with any known cold circuit, or combination of a plurality of cold circuits—regardless of whether pure substances or mixtures circulate therein. 

1. A method for cooling a hydrocarbon-rich fraction, in particular of natural gas, wherein the hydrocarbon-rich fraction is cooled against at least one coolant circuit, the coolant has at least nitrogen, carbon dioxide, methane and/or C₂₊ hydrocarbons, the coolant is compressed by means of at least one turbocompressor having one or more gas-lubricated sliding-ring seals, and as primary seal gas, a substream of the coolant and/or an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor, and as secondary seal gas, nitrogen is fed, characterized in that at least one nitrogen-rich stream is withdrawn at least at times from the coolant circuit.
 2. The method according to claim 1, characterized in that the nitrogen-rich stream is withdrawn at the cold end of the coolant circuit.
 3. The method according to claim 1, characterized in that the nitrogen-rich stream that is withdrawn has a fraction of C₂₊ hydrocarbons of less than 2 mol%.
 4. The method according to claim 1, characterized in that the nitrogen-rich stream is withdrawn as soon as the nitrogen and/or methane content of the coolant circulating in the coolant circuit exceeds a preset threshold value.
 5. The method according to claim 1, wherein the hydrocarbon-rich fraction is liquefied, fed to at least one storage container and from this a boil-off gas fraction is taken off, characterized in that the nitrogen-rich stream that is withdrawn is at least partially added to the boil-off gas fraction and/or is flared off.
 6. The method according to claim 1, wherein the coolant circulating in the coolant circuit is compressed in a single or multistage manner, characterized in that the seal gas mixture that is taken off from the turbocompressor via the primary outlet line of the gas seal is fed to the compressor or the first compressor stage by elevating the seal gas pressure on the suction side.
 7. The method according to claim 1, wherein, as primary seal gas, an external gas or gas mixture having substantially nitrogen and/or methane is fed to the turbocompressor. characterized in that this gas or gas (mixture) is produced from a fraction present and/or occurring within the cooling process.
 8. The method according to claim 1, characterized in that the coolant is expanded at the cold end of the coolant circuit in two stages, wherein the coolant that is expanded in the first stage is separated in a separator into a liquid C₂₊ hydrocarbon-rich fraction and into a gaseous fraction substantially containing solely nitrogen and methane and the liquid C₂₊ hydrocarbon-rich fraction is then expanded to the evaporation pressure of the coolant.
 9. The method according to claim 2, characterized in that the nitrogen-rich stream that is withdrawn has a fraction of C₂₊ hydrocarbons of less than 0.5 mol %.
 10. The method according to claim 7, characterized in that the gas or gas mixture is produced from a substream of the compressed boil-off gas fraction 24 and/or the process nitrogen. 